Industrial gases have achieved widespread utility in industry, and are often supplied to users as cryogenic liquids which are vaporized into the gaseous state for use. The most prevalently used gases are oxygen, nitrogen, argon and hydrogen. Commonly, the gases supplied are of industrial grade with not more than 5000 parts per million of impurities or contaminants. However, the gases required for use in some new semiconductor manufacturing processes must have very low levels of trace contaminants. Increasingly such low levels are required to be under 1 part per million in concentration. Oxygen, for example, now is often required with a purity of 99.9999% by volume. Thus the cryogenic liquids distributed to bulk users can be classified as either of industrial purity or ultra high purity. The ability to consistently deliver an ultra-high-purity cryogenic liquid requires the use of transport trailers and delivery procedures considerably different from those used in the past for industrial purity cryogenic liquids.
A conventional trailer for transporting industrial grade cryogenic liquids has an inner container enclosed in an outer casing. The inner container is generally constructed from type 304 stainless steel or aluminum, typically for a working pressure of 30 psig. Three lateral anti-slosh baffles divide the inner vessel in about four equal volumes. The annular space between the inner container and the outer casing usually contains perlite or fiberglass batting, and is evacuated to develop improved insulating properties. The use points for industrial grade cryogenic liquids are generally located close to the producing plant, so that the thermal insulation achieved with evacuated perlite or fiberglass is adequate to achieve acceptable vaporization losses.
Conventional trailers are equipped with an onboard pump for delivering liquid from the inner container to an external receiver or tank at a use site. The trailer piping is type 304 stainless steel. Valves, gauges, instruments and controls, however, are not necessarily constructed from stainless steel. Bronze, copper and aluminum alloys are utilized. Such components are joined to the piping by threaded or flanged connections.
Conventional trailers have a heat exchanger solely for pressure building in the inner container, typically to a maximum working pressure of 30 psig. The pressure building heat exchanger is heated by natural convection from the atmosphere. The coil is normally constructed of extruded aluminum tubing which is flanged to the stainless steel inlet and outlet piping.
Conventional trailers have a heat exchanger line which leads from a lower port in the inner container to the heat exchanger. A control valve is positioned in this line. At the heat exchanger outlet, a check valve is positioned to prevent back flow into the heat exchanger and into the inner container.
A pressure building line leads from the outlet of the heat exchanger and makes a penetration of the inner container at a level near the top of the container. As used herein, penetration shall mean an opening in the inner container through which a conduit enters and protrudes, or at which a conduit terminates forming an opening to the conduit. The pressure building line terminates in the vapor space within the inner container at a level which is above the liquid level when the inner container is filled to its intended level. Through this line, the vapor or gas which is produced from the liquid vaporized in the heat exchanger is conveyed to the top of the inner container where it serves to build the pressure within the inner container. A check valve, but no control or shut off valve is provided in this line. As used herein, gas shall have the same meaning as vapors.
Branching from the pressure building line is a gas withdrawal line which leads successively to a valve and a first terminal. This terminal has several uses. It is used to connect to a line leading to a recovery unit in an air liquifaction plant to recover vapor generated from the liquid in the inner container. This terminal is also used to connect to a line leading to a ground-mounted pump to recover vapor during cool down of the pump with cryogenic liquid from the trailer. This terminal is also used to connect to a supplementary heat exchanger especially supplied as required for warming cold gas from the inner container and from the heat exchanger. The warmed gas is conveyed to a receiver for purging as described later.
From a second port proximate the bottom of the inner container, a liquid fill line with a control valve runs to a second terminal for connection to an external source of supply of cryogenic liquid. Branching from the lower liquid fill line, from a point between the control valve and the second terminal, is an upper fill line that has a control valve and leads to the top of the inner container. This line penetrates the inner container and runs substantially the full length of the inner container within and along the top of the container. Within the inner container, this line has perforations to distribute liquid or vapor transferred into the inner container uniformly along the length of the container. Liquid is usually transferred and distributed into the trailer through this line to prevent temperature gradients from occurring along the length of the inner container.
Branching from the lower liquid fill line, from a point between the inner container and the control valve, is a liquid delivery line. The liquid delivery line contains successively a control valve, a pump, a check valve and terminates in a third terminal. This delivery line is used to deliver and meter liquid to a customer s receiver.
From the liquid delivery line, from a point between the pump and the check valve, a vapor return line with a control valve runs into the pressure building line. This valve is opened and the vapor return line is used when the pump and the liquid delivery line are warm and are initially placed into service. The vapor return line returns vapor created from liquid vaporized in the warm pump and liquid delivery line to the top of the inner container.
An excess pressure relief line penetrates the inner container at an upper level, and within the inner container terminates at a level which is above the liquid level at the intended full capacity of the trailer. Externally this line leads to rupture disks and spring loaded pressure relief valves.
A tube enters the inner container through a penetration to sense the pressure proximate the top of the inner container. Similarly, another tube enters the inner container through a penetration to sense the pressure proximate the bottom of the inner container. A differential pressure indicator connects across these lines. A pressure gauge is also connected to the line sensing the pressure proximate the top of the inner container.
Typically two lines for liquid level sensing also penetrate the top of the inner container and extend a short distance downward. These lines terminate at different levels within the inner container. Opening the trycock on either of these lines and observing the phase of the fluid which issues determines whether the liquid level is above or below the end of the trycock line. One of these liquid level sensing lines is selected and used to fill the trailer inner container with liquid to the level indicated by that line.
In placing a trailer into service for the first time, or after its wetted volumes and surfaces have been exposed to air and atmospheric moisture, or when the cryogenic liquid to be carried is different from that carried before, it is necessary to purge these prior contents from the wetted volumes and surfaces. Otherwise the prior contents will contaminate the new contents. As used herein, wetted surface shall mean surfaces wetted by, that is, coming in contact with liquid or vapor contents of the trailer during the performance of its functions. By prior contents is meant atmospheric air and moisture which enter an empty trailer, as well as prior cryogenic liquid contents which are different from new contents. Purging is accomplished by flowing through the wetted volumes some of the intended new contents at a temperature greater than 32.degree. F. A purge gas temperature greater than 32.degree. F. is necessary to prevent moisture from freezing out on the wetted surfaces.
Where the intended new contents are of industrial grade purity, the wetted volumes and surfaces in conventional trailers are readily purged to a level where they will not contaminate the new contents. However, if the intended new contents are of ultra high purity, purging of a conventional trailer requires an unduly large expenditure of new contents and an unduly long time.
Welded areas in general have a high degree of surface roughness, porosity and crevices which adsorb, trap and retain prior contents. The wetted surfaces in conventional trailers have a large number of welded areas which are detrimental to maintaining ultra-high-purity contents. In particular, the welds around penetrations of the inner container for ports and entering lines are susceptible to a high degree of surface roughness, porosity and crevice formation. In a conventional trailer, the inner container has nine or more penetrations.
The wetted surfaces in conventional trailers also typically have a roughness conducive to spalling of minute particles which become contaminants. In addition, joints and valve stems are sealed by elastomers which exude contaminants. Thus conventional trailers have shortcomings which preclude their use for transporting ultra-high-purity cryogenic liquids.
Prior to filling an empty external receiver from a cryogenic liquid trailer, it is necessary to purge the receiver to remove the prior contents both from the interior volume of the receiver and from its surfaces. In particular it is desirable to purge out substances that would freeze out on the interior surfaces of the receiver when cryogenic liquid is introduced. Warm purge gas is obtained by warming vapor generated from the cryogenic liquid in the trailer.
To supply warm gas from a conventional trailer for purging a receiver, a supplementary heat exchanger is connected to the first terminal that is, to the terminal at the end of the line branching from the line leading to the vapor space in the inner container. Cold vapor is drawn from the inner container and warmed in the supplementary heat exchanger to at least 32.degree. F. and directed to the external receiver. The supplementary heat exchanger is usually heated by natural convection from the atmosphere. An external heat source is applied to the supplementary heat exchanger if ambient conditions below 32.degree. F. are encountered.
As vapor is removed from the vapor space in the inner container and conveyed to the external receiver for purging, the pressure in the inner container decreases. In order to maintain the pressure within the inner container, liquid is allowed to enter from the bottom of the inner container into the pressure building heat exchanger which is permanently mounted on the trailer. The liquid entering the pressure building heat exchanger is vaporized and conveyed to the top of the inner container. Some of this vapor may flow directly into the supplementary heat exchanger, be warmed and be used as purge gas.
The method of purging an external receiver from a conventional trailer with industrial grade cryogenic liquid has a number of disadvantages. First, a supplementary heat exchanger must be transported to the site of the external receiver. The supplementary heat exchanger itself is normally full of contaminated gas and itself requires purging. Upon the completion of purging of the external receiver, the receiver is disconnected from supplementary heat exchanger and reconnected to the third terminal that is, to the terminal at the end of the liquid delivery line. During this operation, air enters and contaminates the connections. While this is not of great significance in delivering industrial grade cryogenic liquids, this procedure noticeably contaminates an ultra-high-purity liquid. Hence the piping layout, the multiple terminals for external connections and the procedures employed with conventional cryogenic liquid trailer are unsuitable to maintain and deliver an ultra-high-purity cryogenic liquid.
It is an object of this invention to provide a trailer for transporting ultra-high-purity cryogenic liquid and maintaining the purity of the contents.
It is also an object of this invention to provide a trailer for transporting ultra-high-purity cryogenic liquid with the capability of purging an external receiver and transferring the contents of the trailer to the receiver without appreciable contamination of the contents.
It is another object of this invention to provide a trailer which can be rapidly and efficiently purged of prior contents which may contaminate the new contents of the trailer.
It is a feature of this invention that the amount of surface area wetted by the trailer contents is reduced over that of conventional trailers.
It is another feature of this invention that the welded surface area wetted by the trailer contents is reduced over that of conventional trailers.
It is another feature of this invention that the wetted surface areas in the trailer have low roughness.
It is another feature of this invention that the inner container of the trailer has fewer penetrations for ports and lines than inner containers in conventional trailers.
It is another feature of this invention that the connection from the trailer to an external receiver is maintained and not opened when switching from purging of the receiver to filling of the receiver with liquid contents from the trailer.
It is an advantage of this invention that no supplementary external equipment is used to purge an external receiver.
It is another advantage that no atmospheric air or other contaminant is introduced into the contents in changing from purging to filling of an external receiver.